(I) Prior Art Background
Many methods have been used in fracturing connecting rods, that include:
(i) Passing an electron beam along a desired splitting plane as in U.S. Pat. No 3,751,080.
(ii) Providing holes in the fracturing plane through which the fracturing force is introduced as in U.S. Pat. No. 3,994,054
(iii) Using heat treatment or freezing to embrittle the fracture area as in U.S. Pat. No. 4,768,694
(iv) Applying a static or an impulsive force acting perpendicular to the fracture plane as in U.S. Pat. Nos. 4,860,419; 5,115,564; and 5,320,265.
(v) Actuating expanding mandrels using a wedge arrangement as in U.S. Pat. No 5,503,317.
However, most of the known methods for fracturing the connecting rods are based on the same principle: application of an xe2x80x9coutward pressurexe2x80x9d to the crank bore till the generated stresses are high enough to fracture the connecting rod. Some of these methods attempted to overcome the difficulty of fracturing such high strength material by reducing or weakening the cracking area, by using techniques, such as, the cryogenic cooling and the electron beam hardening, which have a deleterious effect on material performance.
Since connecting rods are made of high strength materials, the fracturing force is required to be of big magnitude. The use of big force has a negative effect on the quality of the fractured connecting rod, especially, with large size connecting rods in a high production environment. Despite the improvements, some disadvantages still exist such as: plastic deformation, lack of flexibility in adapting the same technique to different sizes of connecting rods, repeated breakage of force exertion elements of the machine, and poor quality of the fractured connecting rod. Moreover, some techniques are slow, costly, and technically very elaborate.
Before presenting the idea of the current invention, it is necessary to discuss the engineering principles on which the invention stands.
(II) Technical Background
(A) Fracture Mechanics:
Strength failures of load bearing elements can be either of the yielding-dominant (ductile) or fracture-dominant (brittle) types. In case of a cracked element, it may fail due to reaching the plastic collapse or fracture condition. Collapse and fracture are competing conditions, and the one satisfied first will prevail.
High-strength materials are more likely to fail in fracture mode before attaining the plastic collapse strength. Since connecting rods are made of high-strength materials, they generally fail under tensile forces due to reaching the fracture limit state.
Fracture may take place under one of two conditions, namely, plane stress or plane strain, depending on the thickness of the element. In general, connecting rods are thick enough to sustain plane strain fracture. In the presence of a V-notch or a crack, fracture occurs under essentially elastic conditions with a limited plasticity zone at the tip of the crack.
The stress intensity factor (K), is the characterizing parameter for crack extension. For each stress pattern, there is a corresponding value of the stress intensity factor. When the stress intensity factor reaches a certain value, crack propagates and collapse by fracture occurs. That critical value of the stress intensity factor under plane strain conditions, called the Plane Strain Fracture Toughness (KIc), can be considered as a material property characterizing the crack resistance. Thus, the same value of KIc should be obtained for a given material while testing specimens of different geometric shapes and sizes.
Lower temperature and faster strain rate decrease the plane strain fracture toughness for a specific material, while increasing the length of a pre-existing crack or decreasing the fracturing area will increase the stress intensity factor, if all other factors remain unchanged.
(B) Resonance of a Structural System:
The connecting rod, with all movement and rotation constraints imposed on it during the fracturing process, can be viewed as a structural system. Before explaining how to achieve and make use of a resonance condition in this fracturing technique, it is helpful to introduce the following definitions pertaining to an idealized structural system with finite number of degrees of freedom:
Degrees of freedom: the number of independent displacements required to define the displaced positions of all the masses relative to their original positions is called the number of degrees of freedom (DOFs).
Natural mode of vibration: a multi-degree-of freedom system (MDOF) would undergo simple harmonic motion, without a change of the deflected shape, if free vibration is initiated by appropriate distribution of displacements in various DOFs. In other words, for some characteristic deflected shapes, the system would vibrate in simple harmonic motion, and the initial shape would be maintained through out the motion. Each characteristic deflected shape ("PHgr"n) is called a natural mode of vibration of the MDOF system.
Natural vibration properties: the time (Tn) required for a system to complete one cycle of the simple harmonic motion in one of its natural modes is called the natural period of that particular vibration mode. The corresponding natural cyclic frequency of vibration is fn, and the natural circular frequency of vibration is xcfx89n, where:
Tn=2xcfx80/xcfx89n=1/fn.
A vibrating system with N number of DOFs has N natural vibration frequencies xcfx89n (n=1, 2, . . . , N), arranged in sequence from smallest to largest (xcfx891 less than xcfx892 less than . . .  less than xcfx89N), with corresponding natural periods Tn, and natural modes "PHgr"n.
The excitation frequency: the frequency of a harmonic force applied to a system is called the excitation frequency or the forcing frequency.
Damping: the process by which vibration steadily diminishes in amplitude is called damping.
The present invention employs a novel approach to fracture connecting rods. In this process, several factors are used to raise the stress intensity factor in the connecting rod up to the fracture point. Consequently, the use of single big force has been avoided with the application of several small magnitude forces. That eliminates many problems associated with the use of big forces. It also gives better control over the fracturing process, since the contribution of each factor is optimized to achieve the best results. For this process, a stress-riser should be provided in a prior process, using any of the known methods, in order to predetermine the fracture plane.
The present invention utilizes the following factors:
(a) Fatigue: if the stresses in a pre-notched connecting rod fluctuate due to the application of harmonic forces (or any time varying forces), the pre-existing crack (stress-riser) will extend incrementally depending on the range of fluctuation in the stress intensity factor. It is important to notice that the crack growth relates to the change of the stress intensity factor, not to its absolute value. Moreover, as the crack grows, the absolute value of stress intensity factor will increase.
(b) Resonance: during the fracturing process, the connecting rod will be in contact with many elements of the machine. These elements impose movement constraints, called geometrical boundary conditions, to the connecting rod. The connecting rod, with these boundary conditions, represents a distributed mass structural system, with an infinite number of degrees of freedom. However, it can be idealized and analyzed as a system with finite number of degrees of freedoms by using the finite element method.
If a MDOF structural system is subjected to an external force system, where the spatial distribution of the force components is independent of time, it takes a certain deformed shape. This shape does not necessarily resemble any of the natural vibration modes of the system. However, it has the same configuration as one of these natural modes, and with judicious selection of the external forces, the forced deformed shape can present a better approximation to that mode "PHgr"r, which has a natural frequency xcfx89r. In most cases, "PHgr"r is one of the first few natural modes.
If the force components have the same sinusoidal time variation, with a frequency that is the same as or close to the natural frequency xcfx89r, a resonance condition occurs. Consequently, the fluctuation range of the stress intensity factor and its maximum value increase substantially. The crack extends, and fracture may occur, depending on the relative magnitudes of stress intensity factor and material fracture toughness.
The aforementioned principle is applied in the present invention, where two harmonic forces, with the same amplitude and a phase angle of 180xc2x0, are applied simultaneously to two sides of the connecting rod. The two forces act along a straight line parallel to the predetermined fracture plane and perpendicular to the axis of the bore cylindrical surface. Moreover, the clamping arrangement allows free deformation of major part, centered on the V-notch, of each of the two webs defining the bore.
A suggested method, to apply the two harmonic forces, is to transform the rotary motion generated by a hydraulic motor to a couple of rotary motions, one clockwise and another anti-clockwise. These rotary motions can be transformed to linear motions using cam means, which transfer the pressure to the connecting rod through two contacts. The use of a single motor will guarantee that there is no time lag between the two equal but opposite forces.
(c) Pre-stressing forces: three pre-stressing forces are applied in the present invention. The primary one is applied by moving an upper jaw, which is part of the clamping arrangement, in a direction perpendicular to the predetermined fracture plane and away from it. This force works to eliminate the compression stress zones created by the harmonic forces and to decrease the deformation due to its application, giving more rigidity to the system. Eliminating the compression zones is favorable, since they deviate the fracture from predetermined plane.
The secondary pre-stressing forces are two static forces, equal in magnitude and acting on the same straight line in opposite directions, toward the bore axis. The same mechanism used to apply the harmonic forces can be used to apply these forces. Firstly, the two contacts advance till they slightly press the part applying the secondary pre-stressing forces, and secondly, they move forward and backward applying the harmonic forces.
These forces have clamping and damping actions. However, since they act in the same opening loading mode as the primary force, the stresses due to all of them can be superimposed. This will facilitate further reduction of the high magnitude fracturing forces.
Under the effect of the external forces, the deformed shape of the connecting rod changes with time. However, during each cycle, it passes through maximum and minimum deformation positions at time instants Tmax and T0, respectively. The stress intensity factor corresponding to the harmonic forces has a maximum value at the maximum deformation position. Both of T0 and Tmax can be determined analytically by knowing the characteristics of the forces.
(d) Dynamic force: a dynamic force is finally applied at a time instant Tf, by increasing the primary pre-stressing force suddenly as an impulsive force at Tf, or at a slower rate within a period centered on Tf. The time instant Tf to be determined by performing several simple tests, by applying the fracturing force during different cycles at different time instants such as T0 (minimum deformation) or Tmax (maximum KI) and comparing the quality of the fractured connecting rods.
For example, if T0 comes after 0.10 seconds from the beginning of each cycle, and the natural vibration period is 0.25 second, a test can be performed by applying an impulsive force at T0 during the cycle 101 after 25.10 seconds from the beginning of the harmonic excitation. In another test, a dynamic force is applied during a period starting from the instant 25.05 seconds, and ending at the instant 25.15 seconds, measured from the beginning of the excitation. Similar tests are performed by applying the fracturing force during different cycles, and at different time instants, and by comparing the quality of the fractured connecting rods, Tf is identified. However, a longer period before applying the dynamic force, increases the fatigue effect.
All of the external forces used in the aforementioned factors are in the same loading mode and, generally, stress the connecting rod within the linear elastic regime. Thus, the stress intensity factor due to their collective effect, xcexa3KI, is obtained by adding the values of KI, that correspond to each one of them, if applied individually. Fracture occurs when xcexa3KI=KIc.
The flexibility of the external force system used in the current invention, makes the technique suitable for the wide variety of connecting rods types and sizes, starting from those intended for light duty applications such as lawnmowers and outboard marine engines, up to the most powerful combustion engines.